1. Field of the Invention
The present invention relates to an etching method for etching a material to be etched, in other words, a sample, by exciting an etching gas to a plasma state; and specifically, an etching method suitable for selectively etching a sample with respect to an organic material, wherein the underlying substance of the film to be etched is an organic film.
2. Description of the Related Art
Techniques for etching semiconductor devices such as microwave plasma etching, reactive ion etching and the like have been known. In these etching techniques, an etching gas is excited to a plasma state using a radio-frequency electric field by parallel-plate electrodes or cyclotron resonance, and the material is etched. These etching techniques have also been used as techniques for etching a nonvolatile material used in ferroelectric memories. For example, as a method for etching an Al film as the material to be etched, the plasma of a Cl2-based mixed gas containing BCl3 is generally used as the etching gas. As a method for etching an Au film as the material to be etched, as a mixed gas of halogen gases other than a mixed gas of CF4 and O2, or CF4, or an inert gas such as Ar, is used (e.g., refer to Japanese Patent Application Laid-Open No. 6-84839 (patent document 1) and Japanese Patent Application Laid-Open No. 6-112169 (patent document 2).
In the etching of the material to be etched, it is required, as the etching performance, that the material to be etched is selectively etched. Specifically, when a material such as a photoresist film, an oxide film or a nitride film is used as a masking material, it is required that the material to be etched is selectively etched against the masking material. In other words, it is required that there is a large selection ratio between the etching rate of the material to be etched to the etching rate of the masking material. Similarly, it is also required that the material to be etched is selectively etched against the underlying material. In other words, it is required that there is a large selection ratio between the etching rate of the material to be etched to the etching rate of the underlying oxide film.
In etching techniques conventionally used, for example as shown in FIG. 1, when an Al film 3 in a material to be etched is etched using a photoresist film 4 as a mask, wherein an oxide film (SiO2 film) 2 is formed on an Si substrate 1 and the Al film 3 is formed thereon, a Cl2-based mixed gas is used, and plasma is formed from the gas to etch the Al film 3, which is the material to be etched. At this time, the underlying oxide film 2 is also etched because it is similarly exposed to the plasma.
The Al film is etched since aluminum chloride is mainly formed by the reaction with chlorine radicals and chlorine ions formed from the Cl2-based mixed gas. The underlying oxide film exposed to the plasma is also etched since silicon tetrachloride is mainly formed by the reaction with the chlorine radicals and the chlorine ions.
At this time, the bonding energy of an Al—Al bond composing the Al film is 40 kcal/mol, the bond energy of an Al—Cl bond composing aluminum chloride, which is the reaction product, is 118 kcal/mol, the bond energy of an Si—O bond composing the oxide film, which is the underlying substance, is 192 kcal/mol, and the bond energy of an Si—Cl bond composing silicon tetrachloride, which is the reaction product, is 77 kcal/mol. The chemical reaction proceeds when the bond is broken and another bonding form is produced by applying energy larger than the bond energy.
In this case, since the bond energy of the Si—O bond of the oxide film of the underlying material is larger than the bond energies of the Al—Al, Al—Cl and Si—Cl bonds, etching of the Al film proceeds easily than the oxide film. In other words, the etching rate of the Al film is higher than the etching rate of the oxide layer, and the Al film can be selectively etched against the oxide film.
However, if the underlying material is an organic film, it is difficult to etch an Al film, which is a material to be etched, selectively against the organic film. For example, as shown in FIG. 2, in a material to be etched wherein an organic film 5 is formed on an Si substrate 1 and an Al film 3 is formed thereon, when the Al film 3 laminated on an organic film 5, which is an underlying material, is etched using a photoresist film 4 as a mask, the Al film 3 is etched because aluminum chloride is mainly formed by the reaction with chlorine radicals and chlorine ions in plasma formed from Cl2-based mixed gas. Since the underlying organic film 5 is also exposed to the plasma, it is etched because carbon tetrachloride is mainly formed by the reaction with chlorine radicals and chlorine ions. Since the bond energies of the C—C, C—H and C—F bonds composing the organic film 5, which is the underlying material, are 144 kcal/mol, 81 kcal/mol and 107 kcal/mol, respectively, and are smaller than the bond energy of the Si—O bond when the underlying material is an oxide film, which is 192 kcal/mol, the organic film 5 can be easily etched than the oxide film. Specifically, the selection ratio of the Al film against the underlying material is lowered when the underlying material is changed from the oxide film to the organic film. The generally known selection ratio of the Al film against the underlying organic film is a value of 2 or less.
Furthermore, although a halogen gas is generally used for etching nonvolatile material, such as Au and Pt, since the saturated vapor pressure of the reaction product thereof is lower than the saturated vapor pressure of a photoresist, which is the masking material, and an oxide film or an organic film, which is the underlying material, in the etching of a nonvolatile material, it is difficult to selectively etch the photoresist of the masking material and the oxide film or the organic film of the underlying material. The generally known selection ratio of Au or Pt, which is a nonvolatile material, against the underlying oxide film or organic film is 0.2 to 0.8, which is less than 1.